The invention relates to a combination electrode for the electrochemical restoration of corrosion-damaged reinforced concrete and a method of controlling the same.
Mineral based construction materials used in construction technologies, such as, concrete, cement, or mortar, are customarily reinforced with steel elements that are embedded for this purpose into the mineral-based construction material. In the case of a properly manufactured concrete element, these steel elements are coated with such a thick concrete layer that the steel element is, as a rule, permanently protected against corrosion. The corrosion protection of the concrete layer is based on the alkalinity of the water that is found in the pores of the concrete and is given when the pH-value is greater than 12.5. Under these conditions, a thin, adherent oxide film forms on the surface of the steel, which in practice completely prevents a corrosion of the steel element. For this reason, reinforced concrete can be used for external components that are subject to weather conditions. Under unfavorable conditions, however, especially in the case of faulty construction work and, particularly, if the structure is exposed to salts, the corrosion protection of the reinforcement can be impaired. Under such conditions, the concrete undergoes carbonatization when carbon dioxide from the air reacts with the alkaline components of the cement. The pH-value then drops, thereby reducing or eliminating the corrosion protection.
Another frequently occurring cause for corrosion is the ingression of chlorides into the concrete, for example, when concrete components are used as a roadway or in the vicinity of a roadway where de-icing salt is used. Both corrosion processes begin on the surface of the concrete and continue to work their way into the interior of the concrete to the steel components embedded therein, eventually causing the oxide film to dissolve. The concrete is destroyed by the spalling that results from the voluminous build-up of corrosion products. In order to prevent this, the concrete containing the chloride must be replaced, or the chloride itself must be removed from the concrete.
Various methods are known for replacing contaminated material. These methods are physical methods that entail the removal of contaminated material and its replacement with fresh concrete. This, however, is a costly solution because of the effort involved. Furthermore, it is only applied if an area has already been recognized as damaged.
For this reason, a method is proposed in which the chloride is removed electrochemically from the concrete by means of ionic migration. In this way, the chloride concentration in the concrete is drastically reduced, so that it is no longer necessary to replace the concrete. The method described by J. E. Slater in Materials Performance, 1976 (pp. 21-26) applies an electrical potential between the interior reinforcement and a surface electrode that is submerged in a liquid electrolyte provided on the surface of the concrete. The surface electrode thereby forms the positive pole of the electrical field which causes the negatively charged chloride ions that are contained in the concrete to migrate through the concrete, exit the concrete, and reach the electrolyte. The chloride ions are oxidized to gaseous chlorine in the electrolyte at the positive electrode, or they react chemically with components that are contained in the electrolyte. The method proposed by Slater, however, has several disadvantages: For one, this method requires that voltages between 100 and 120 V be applied. Such high voltages, which must be applied over a period of 24 hours in order to remove approximately 90% of the chloride from the concrete, are unacceptable for technical safety reasons. Furthermore, the costs thereby incurred can surpass even those of conventional technologies. For another, the liquid electrolyte used is restrained by dams that must be built on the surface of the concrete. The method can therefor be applied only to horizontal concrete surfaces. This restriction results in signficant expenditure for the preparation of the chloride removal, and moreover greatly restricts the applicability of the method. A further disadvantage of this method is that it does not particularly take into account non-homogeneities that are present in the reinforced concrete (concrete covering, concrete density, moisture content, reinforcement density relative to the surface of the concrete). When treating large-surface construction components these non-homogeneities can lead to significant variations in process intensity.
With respect to the latter mentioned problem, DE 4 229 072 A1 proposes an electrolyte reservoir with a resistance of R>>0, so that partial overheatings, e.g., with a single protruding reinforcement with the then resulting small RConcrete, are compensated by the RElectrolyte. A disadvantage of this method is that the process peaks are merely automatically cut off, without feedback control occurring in the case of a partial hypofunction (Rconcrete large).
U.S. Pat. No. 5,228,959 (Miller; issued 1993) proposes as the electrolyte an adherent layer that can be applied to vertical surfaces or even to the undersides of concrete elements and that adheres to the same. An electrode that constitutes the positive pole of the electrical system is embedded in the adherent layer. When the process is completed, that is, when the level of chloride contamination has been reduced to a desired level, the adherent layer, as well as the electrode, is removed from the concrete surface. The individual steps of the method comprise applying a removable adherent electrolyte layer onto the surface of the concrete; applying a d.c. voltage between the reinforcement and the electrode, to cause the negative ions to migrate; interrupting the applied voltage and removing the adherent layer when the desired level of ion migration has been achieved; periodically measuring the difference in potential between the reinforcement and the reference electrode and temporarily interrupting the d.c. voltage when the difference in potential indicates the formation of hydrogen. Although this method offers a number of advantages when compared to the method proposed by Slater, it does have the disadvantage that the electrochemical system can be used only once; re-use is impossible. Even though it states in U.S. Pat. No. 5,228,959 that the used material can be easily and inexpensively disposed of, this is no longer acceptable from the viewpoint of increasingly restrictive environmental standards and increasing disposal costs. Furthermore, the release of chlorine that is a by-product of the method is extremely harmful to the environment.
This method has been modified in several patent applications. EP 0 398 117 A1, for example, proposes a cellulose pulp as the material for the adherent electrolyte layer. The cellulose pulp can be premixed in a nozzle with water or another solution, such as calcium hydroxide, and sprayed onto the surface of the area to be treated. This patent application also proposes that a ferriferous material that is reactive in the presence of chlorine be used for the electrode, in order to minimize the release of free chlorine into the environment. This has the advantage of releasing less chlorine into the environment, but the disadvantage of not completely eliminating the formation of chlorine. Furthermore, the electrode disintegrates in the course of the application of the method so that, for this reason alone, the method can be used only once.
What is needed, therefore, is an electrochemical method of restoring corrosion-damaged reinforced concrete that is economical in terms of apparatus, energy demand, and time required to complete the restoration, and is safe to use. What is further needed is such a method that can be used on irregular, vertical, and downward facing surfaces of structures. What is yet further needed is such a method that is adjustable to the variations present in the properties of the area to be treated. What is still further needed is such a method that is environmentally non-polluting. And finally, what is needed is apparatus for carrying out and controlling the method, wherein the apparatus is usable for a multiple number of applications.